The appearance of commercial color photocopiers in the 1970's presented counterfeiters around the world with a powerful, widely accessible tool for creating passable reproductions of currency and other security documents such as treasury bills and airline tickets.
The proliferation in recent years of inexpensive color scanning and printing technology for personal computers has presented treasury departments with a new challenge with respect to currency counterfeiting. For example, an inexpensive system including a 720.times.720 color ink-jet printer with a 300 DPI flatbed scanner can be used to create color reproductions that exceed the quality of color photocopiers costing more than a hundred times as much.
This development has brought about a need for embedding a serial number, on the order of 32 bits, in an image printed by a printing system including an ink-jet printer in a manner that does not adversely affect image quality. At the same time, the encoding should not require extensive or expensive computational resources, since the goal would be ultimately to integrate the encoder into the printer itself. Also, for analysis, the bits should be detectable after digitizing by a flatbed scanner of typical consumer resolution, currently 600 DPI or less.
Enabling such an ink-jet printer to encode its serial number or some other identifying bit string onto its printed output differs fundamentally from the analogous problem for a photocopier. The dithering algorithm is not integral to the printer but is handled by software on a host computer or a plug-in card. An ink-jet printer handles data for only a small number of lines, corresponding to one or two passes of the printing head, at one time. A consumer ink-jet typically prints a quarter-inch BAND across an 8.5-inch path length in a single pass. Ideally, any technique should require image data from only one pass at a time.
Decoding the embedded serial number may be complicated by the nonlinear modifications introduced into the document by the print/scan sequence. Ink-jet printing subjects the document to be reproduced to nonlinear modifications not necessarily introduced by photocopying; these modifications take the form of spatial resolution lost to dithering in order to enhance the color depth obtainable from the four to seven colors of ink in its palette. If this output is then scanned, to examine the document for an encoded serial number for example, the creation of its RGB representation introduces further modifications. Scanning also typically introduces some translation and rotation of the image.
In terms of data hiding, this situation differs from the traditional information hiding problems. Typically for images, data hiding techniques are designed with the understanding that the quality of a test image might be largely degraded compared to the original unaltered host image in terms of signal-to-noise ratio through perceptual coding methods such as JPEG; that arbitrary resampling might have been done through scaling; and that cropping is a possibility. Most commercial systems also presuppose that a test image presented to the decoder has not been rotated with respect to the host image; often such systems require the test image to be untranslated as well. Furthermore, it is often assumed that the test image will be in a similar color/luminance space--RGB v. CMYK, for example--as the original host image.
By contrast, data hiding for detecting counterfeiting of security documents is constrained by an almost complementary set of circumstances. An offender is motivated to create a reproduction that looks as much as possible like a legitimate document before trying to pass it. Thus the quality of the reproduced image that would serve as a test image is usually excellent, and the size and scale of the reproduction is fixed.